1. Field of the Invention
The principles of the present invention generally relate to backlight units. More particularly, the principles of the present invention relate to a method for driving a backlight unit facilitating the stable activation and/or deactivation of the backlight unit and preventing a malfunction in driving the backlight unit.
2. Discussion of the Related Art
Generally, display devices such as plasma display panels (PDPs), field emission displays (FEDs), light emitting diodes (LEDs) and liquid crystal displays (LCDs) are known as flat panel displays. Based on the manner in which they express images, flat panel displays can be classified as either light emission-type flat panel displays or light receiving-type flat panel displays. PDPs, FEDs, and LEDs are light emission-type display devices and LCDs are light receiving-type display devices. Therefore, and to express images, an LCD panel within an LCD device must receive light that is generated by an external light source. In many cases, such a light source is provided within a backlight unit.
Backlight units incorporated within LCD devices must usually have such features as a high brightness, a high operating efficiency, uniform distribution of brightness, long life span, thin profile, light weight, low price, low power consumption, and the like. Backlight units incorporated within LCD devices for notebook computers, for example, are generally required to have a low power consumption, a high efficiency, and long life span. Backlight units incorporated within LCD devices for televisions or monitors of personal computers, for example, are generally required to have a high brightness. Depending on the location of the light source with respect to the LCD panel, backlight units can be generally classified as either direct-type and edge-type.
Edge-type backlight units generally include a lamp provided along at least one lateral side of a light-guide plate that is disposed beneath an LCD panel. The light-guide plate diverts light emitted by the lamp from the lateral side to a backside of the LCD panel to uniformly transmit the emitted light into the LCD panel. Direct-type backlight units generally include a plurality of lamps provided beneath a lower surface of a light-diffusion sheet that is, in turn, disposed beneath an LCD panel.
Because many backlight units are required to emit light at a high brightness, lamps within direct- and edge-type backlight units can be provided as cold cathode fluorescent lamps (CCFLs). The CCFL includes a step-up transformer to generate the high voltage necessary to initiate and maintain discharge within the CCFL from a low AC voltage having a frequency of a few tens kHz. The low AC voltage is generated by an LC resonant inverter and has a sinusoidal waveform. While the LC resonant inverter is structurally simple and highly efficient, a plurality of CCFLs connected in parallel cannot be driven by one inverter. Accordingly, both direct- and edge-type backlight units employing multiple CCFLs undesirably require a corresponding number of inverters.
To overcome the aforementioned disadvantages of incorporating CCFLs within backlight units while satisfying the same backlight unit requirements, external electrode fluorescent lamps (EEFLs), i.e., lamps with electrodes on the outside of the lamp tube, have been developed. Backlight units incorporating EEFLs can generate light having a high brightness (i.e., a few ten thousands of cd/m2) using an RF driving frequency of only a few MHz. A known direct-type backlight unit includes a plurality of EEFLs disposed on a light reflecting plate. Moreover, when connected in parallel, EEFLs can be driven using one inverter (i.e., a transformer). When driving EEFLs using connected in parallel, current flowing within respective lamps is equal to the sum of each current within the lamps. Consequently, the total current within the system can be very large. When such a large current contacts the human body, the result can be fatal. Accordingly, a limit current circuit (LCC) is used to deactivate the EEFLs when the EEFLs are in contact with the human body.
FIG. 1 illustrates a block diagram of a related art backlight unit driver. FIG. 2 illustrates waveforms associated with the related art backlight unit driver shown in FIG. 1.
Referring to FIGS. 1 and 2, an enable signal for driving a lamp assembly 17 is input to a controller 11. The enable signal can be generated within the driver or be supplied from an external source. In response to a constant high state of the enable signal (indicating that a lamp assembly 17 is to be driven), the controller 11 generates a pulse width modulated (PWM) signal and outputs the PWM signal to a FET 13, which also receives an externally input DC voltage Vin. Specifically, the FET 13 includes four transistors connected in parallel and one capacitor. Accordingly, and upon receipt of the PWM signal and the input voltage Vin, the FET 13 generates and outputs a positive DC square wave voltage every odd pulse of the PWM signal and a negative DC square wave voltage every even pulse of PWM signal. Therefore, the FET 13 alternately generates and outputs positive and negative DC square wave voltages in response to the sequential pulses of the PWM signal.
A transformer 15 boosts the output DC square wave voltage by a predetermined amount and outputs the boosted voltage to the lamp assembly 17, which includes a plurality of EEFLs connected in parallel. Accordingly, the transformer 15 outputs a boosted voltage having a substantially constant voltage during predetermined periods. Due to the parallel connection of the lamps within the lamp assembly 17, only one transformer 15 is needed to drive the lamp assembly 17.
An LCC protection circuit 19 is disposed between the transformer 15 and the lamp 17 and detects voltage and current characteristics, Vo and Io, respectively, of the transformer 15 or the lamp assembly 17. While the boosted voltage output by the transformer 15 has electrical characteristics associated with AC voltage, the boosted voltage output by the transformer 15 is rectified and converted into a DC voltage before it is provided to the LCC protection circuit 19. The manner in which the conversion is accomplished will not be discussed herein as such an operation is widely known to those in the art.
Thus, because the driver discussed above with respect to FIGS. 1 and 2 enables EEFLs to be driven according to a boosted voltage having a square waveform with a substantially constant voltage during predetermined periods, EEFLs within the lamp assembly 17 may generate light having a uniform brightness.
FIG. 3A illustrates electrical properties of a related art backlight unit driver in a normal operation mode. FIG. 3B illustrates electrical properties of a related art backlight unit driver in an abnormal operation mode.
Referring to FIG. 3A, under normal driving operations, voltage Vo and current Io, as detected by the LCC protection circuit 19, are substantially constant. However, and with reference to FIG. 3B, when either the transformer 15 or the lamp assembly 17 is contacted by an external object (e.g., the human body), the voltage Vo or the current Io can rise. If either the voltage Vo or the current Io rise too quickly, the LCC protection circuit 19 recognizes the rise as a malfunction and transmits an alarm signal to the controller 11. In response to the transmitted alarm signal, the controller 11 ceases generating the PWM signal, the transformer 15 is prevented from supplying the boosted voltage to the lamp assembly 17, and the lamp assembly 17 is deactivated.
Referring back to FIG. 2, when the lamp assembly 17 is initially driven, the transformer 15 outputs a boosted voltage having a transient overvoltage or overcurrent (i.e., effects of a naturally occurring overshooting phenomenon where the voltage suddenly varies). The magnitude of the overshooting phenomenon depends upon the output of the transformer 15 and the electrical capacity of the lamps within the lamp assembly 17. Specifically, wall charges are not charged within each lamp of the lamp assembly 17 after the boosted voltage is initially output by the transformer 15 and before the lamps produce electrical discharges to emit light. Moreover, lamps within the lamp assembly 17 have a net capacitive load before they emit light but have both capacitive and resistive loads after they emit light. The resistive component of the load produces oscillating attenuation affects. As a result, the overshooting phenomenon occurs naturally, before the lamps within the lamp assembly 17 produce electrical discharges to emit light. Therefore, as the lamps are driven over time, the overvoltage or overcurrent associated with the overshooting phenomenon is reduced and the voltage drop within each lamp decreases to a normal voltage. Moreover, the impedance of the transformer 15 cannot be adjusted to suppress the overvoltage or overcurrent.
Thus, when the transformer 15 initially generates and outputs a boosted voltage that induces the overshooting phenomenon, the boosted voltage is output to the lamp assembly 17 as well as to the LCC protection circuit 19. The LCC protection circuit 19 then erroneously registers the initially generated boosted voltage as a malfunction caused by contact with a human body and transmits an alarm signal to the controller 11, wherein the controller 11 erroneously ceases generating the PWM signal, ultimately deactivating by lamp assembly 17 by preventing the transformer 15 from outputting the boosted voltage thereto. As discussed above, however, the aforementioned overshooting phenomenon, generated upon initially driving the lamp assembly 17, is not a malfunction of the backlight unit driver. Rather, it is a naturally occurring and desirable phenomenon (i.e., each EEFL spontaneously discharges due to the overvoltage or overcurrent associated with the overshooting phenomenon and generates light having a higher brightness more efficiently).
Because the LCC protection circuit 19 of the related art backlight unit driver erroneously registers the natural overshooting phenomenon as a malfunction, the related art backlight unit driver erroneously deactivates the lamp assembly 17. Further, once the lamp assembly 17 is deactivated, the enable signal shown in FIG. 2 must be reapplied to the controller 11 to reactivate the lamp assembly 17. Accordingly, the related art backlight unit driver does drive the lamp assembly 17 in a stable manner.